1. Field of the Invention
The present invention relates to an image reading apparatus of the type in which an optical image on a document original is formed on a solid state imaging device by means of an optical imaging means, and the imaging device converts the information of the optical image into corresponding electrical signals. More particularly, the invention relates to an image reading apparatus succeeding in substantially eliminating the blur in the image, which arises from a positional variation of the surface of a document original relative to the solid state imaging device, and the distortion in the image, which arises from the curved surface or the undulation of the document original.
2. Discussion of the Related Art
A conventional image reading apparatus used for facsimile devices, digital copying machines, scanners for personal computers, and the like is shown in FIG. 18.
In the apparatus, as shown, a document original 103 is located on a platen glass (not shown), which constitutes substantially the top surface of a housing 101. A light source 105, a mirror 107, and a lens 109, which cooperate to form a reduction-magnification optical system, and a solid state imaging device 111, such as a charge coupled device (CCD), are disposed within the housing 101.
In the image reading apparatus, light emitted from the light source 105 is reflected by the original 103, and the reflected light passes through the reduction-magnification optical system to form an image of the original on the solid state imaging device 111. The imaging device converts the received light image information into corresponding electrical signals.
Another type of the conventional image reading apparatus is roughly illustrated in FIG. 19. In this apparatus, a light source ].05, and a rod lens array 113, which cooperate to form an equal-magnification optical system, and a contact type sensor 115, are disposed within a housing 101 with a platen glass (not shown) substantially constituting the top surface thereof.
In the image reading apparatus, light emitted from the light source 105 is reflected by the original 103, and the reflected light passes through the equal-magnification optical system to form an image of the original on the contact type sensor 115. The sensor converts the received light image information into corresponding electrical signals.
In the two image reading apparatuses as mentioned above, the processing of signals that are read is based on the presumption that the original entirely contacts with the platen glass. Accordingly, the image reading apparatuses involve the following problems. In a case where the object whose image is to be read is a relatively thick object, such as a book, a portion of a page of the book near the back of the book is curved upward from the platen glass. When the book is put on the platen glass and an image on the page is read, characters, for example, on the corresponding portion of the read image are reduced in size. To avoid this, the image signals obtained must be corrected.
To solve the above problem, there is proposed an image reading technique as shown in FIG. 20. In the figure, a light source 25 emits a line beam in the direction of a spread of a book 5 (horizontal direction in FIG. 20). A cylindrical lens 26 shapes the light beam from the light source 25 into a desired shape of beam. The line beam light emitted from the combination of the light source and the cylindrical lens is reflected by the pages of the book 5, and is imaged on an optoelectric transducing element 28, through a lens 27. A profile of a contour of the page surface is calculated using the output signals of the optoelectric transducing element. The image signals from the curved portion on the pages near the back of the book (FIG. 20) are corrected so as to extend the image thereon in the horizontal direction. As a result, the reproduced image on the curved portion of the book 5 is a little distorted.
The reproduced image, even after extension process for correction, still contains a little blur in the curved portion near the back of the book 5, which is set rising to the platen glass. The reason follows. The extension process for correction is based on the information representative of the surface profile of the original that is read by the line beam light. However, the focal distance of the read sensor to the thick original, or the book, is not adjusted. Accordingly, the surface profile information contains image data read at the improper focal distance.
The correction technique is well adaptable for the image reading apparatus based on the reduction-magnification optical system, but is not suitable for the image reading apparatus of the type which employs the equal-magnification optical system for the size-reduction purposes, because use of the correction technique inevitably brings about increase of the size.